This invention relates to electrochemical cells with aqueous alkaline electrolytes and a negative electrode or counter electrode in contact with a cell casing or cup. Electrochemical cells can be used as batteries to provide energy to operate electronic devices. Electrochemical cells, particularly prismatic cells and button cells, are suitable for applications including hearing aids, sensors, computers, calculators, watches and other devices.
Embodiments include zinc/metal oxide cells, with positive electrodes containing one or more metal oxides such as silver oxide, manganese dioxide, nickel oxyhydroxide, silver copper oxide and silver nickel oxide. Other embodiments include cells with catalytic electrodes, such as electrodes that reduce oxygen or generate oxygen or hydrogen, such as fuel cells, metal-air cells, oxygen generating cells and hydrogen generating cells. The invention also relates to methods for forming an electrochemical cell, preferably including providing an electrode casing with a coating that reduces discoloration of a surface exposed to the ambient atmosphere.
Other embodiments of electrochemical cells include alkaline cells having a metal, such as zinc, as an active negative electrode material and a positive electrode including a metal oxide or metal dioxide such as silver oxide and manganese dioxide. Cells containing a metal oxide and/or metal dioxide are usually noted for good voltage stability during discharge. Zinc/metal oxide and zinc/metal dioxide cells typically include an alkaline electrolyte and an electrolyte permeable separator film located between the negative electrode and the positive electrode. Examples of such cells are described in U.S. Pat. Nos. 4,405,698, 6,794,092, 6,759,166, and 6,723,469, as well as U.S. patent application Ser. No. 11/731,361 filed Mar. 30, 2007, all fully incorporated herein by reference.
Some electrochemical cells can be used to generate gases, such as oxygen and hydrogen. One type of electrochemical battery cell has a metal, such as zinc, as an active negative electrode material within the cell and uses a gas from outside the cell, such as oxygen contained in the air, as an active positive electrode material. An example of such a cell is an alkaline electrolyte zinc/air cell. When an external electrical circuit is completed, oxygen in air that enters the cell is reduced at the catalytic positive electrode to produce hydroxyl ions. The hydroxyl ions migrate to the negative electrode where they oxidize the zinc, producing electrons that flow through the external circuit. An example of an alkaline zinc/air cell button cell is disclosed in U.S. Pat. No. 6,602,629, which is hereby incorporated by reference. Electrochemical cells similar in design to an alkaline zinc/air cell can also be used to produce oxygen or hydrogen. In an oxygen generating cell, the catalytic electrode is the oxygen generating electrode, and the counter electrode is a metal oxide (e.g., manganese dioxide, silver oxide, mercuric oxide or nickel oxyhydroxide) rather than zinc. When an electric current is forced to flow through the cell, the metal oxide is reduced to a lower oxidation state, and oxygen is evolved at the oxygen generating electrode. Such a cell is described in U.S. Pat. No. 5,242,565, which is hereby incorporated by reference. In a hydrogen generating cell, the catalytic electrode is the hydrogen generating electrode; the active material of the counter electrode can be a metal such as zinc. When oxygen is excluded from the cell and the cell is short circuited, hydrogen is produced. Examples of hydrogen generating electrochemical cells are disclosed in U.S. Pat. Nos. 5,242,565, 5,707,499 and 6,060,196, all of which are hereby incorporated by reference.
The generation of hydrogen within alkaline electrochemical cells during periods of storage and non-use can be detrimental, particularly for cells in which a portion of the cell housing serves as a current collector. Mercury can be added to these cells to reduce such undesirable hydrogen gassing; however, the addition of mercury is undesirable for health and environmental reasons. Efforts have been made to eliminate mercury, but the cells are still more susceptible to gassing without added mercury.
Electrochemical cells, particularly prismatic cells and button cells, can comprise two electrodes, each in contact with an electrode casing that can serve as a current collector therefore. Depending on the cell construction, a casing can be referred to as a negative electrode casing, a positive electrode casing, a catalytic electrode casing or a counter electrode casing. Such casings can have similarly shaped bodies such as a cup or a pan, each with a closed end and an open end generally opposite the closed end. Of course, metal-air, fuel and gas generating cells typically include one or more orifices in the catalytic electrode casing to allow ingress or egress of gases such as oxygen, carbon dioxide and hydrogen. Prismatic cells and button cells can be sold as single cell batteries not having any jackets or labels covering the external surfaces of the cells. Accordingly, any portion of the cell's internal components, such as liquid electrolyte or salt, that contact the external surfaces of the cells, such as through leakage, is present on an external surface of the one or more cell casings. The presence of leaked components on the external cell surfaces can lead to corrosion thereof. In some instances, the ambient atmosphere can also lead to discoloration of the external cell surfaces. It is also desirable to maintain good electrical contacts in other single cell batteries as well as cells used in multiple cell batteries.
Various attempts have been made to minimize discoloration of external surfaces of electrochemical cells and to improve the leakage resistance of cells which can affect tarnishing of the cells external surfaces. For example, clad metals, such as triclad nickel/stainless steel/copper (Ni/ss/Cu), have been utilized as negative electrode casings for button cells including zinc/silver oxide, zinc/manganese dioxide, zinc/nickel oxyhydroxide and metal-air cells. The nickel plated outer layer is believed to provide an attractive appearance while resisting discoloration. The stainless steel provides strength and the copper inner layer has desirable electrical conductivity, and provides a continuous coating over the stainless steel that can be formed into a desired shape without cracking to expose the stainless steel layer therebeneath. The copper layer is also readily plated with zinc when contacted by a negative electrode containing an alkaline electrolyte and zinc as an active material.
U.S. Pat. No. 6,830,847 relates to a zinc/air button cell comprising a cathode casing and an anode casing wherein the anode casing is inserted into the cathode casing. The anode casing is formed of multi-clad metal layers, for example nickel/stainless steel/copper. A reportedly protective metal is plated on the exposed peripheral edge of the anode casing. The protective metal is desirably selected from copper, tin, indium, silver, brass, bronze or gold. The application of protective metal covers the multi-clad metals exposed along the peripheral edge surface. The protective metal is also desirably plated onto the portion of the outside surface of the anode casing abutting the insulating material placed between the anode and cathode casing. Application of the protective metal to the anode casing peripheral edge reportedly eliminates the potential gradients caused by exposure of the different metals comprising the multi-clad material. This reportedly reduces the chance of electrolyte leakage which can be promoted by secondary reactions occurring along the anode casing peripheral edge.
U.S. Patent Application Publication No. 2003/0211387 relates to a galvanic element with an alkaline electrolyte and a zinc negative electrode, in a housing in the form of a button cell, where at least the outer surface of the cell's cap is coated with a Cu—Sn-alloy containing no nickel or with a Cu—Sn—Zn-alloy containing no nickel. The alloy contains about 20% to about 90% Cu, preferably about 50% to about 60% Cu, with the remainder being Sn, or about 50% to about 60% Cu and about 25% to about 35% Sn, with the remainder being Zn.
U.S. Patent Application Publication No. 2006/0246353 relates to an electrochemical cell with a zinc-containing negative electrode, an aqueous alkaline electrolyte and a cup-shaped metal negative electrode casing in contact with the negative electrode. The negative electrode casing is formed from a substrate that is substantially free of copper and at least those portions of the surface of the negative electrode casing in the seal area and the current collector area are coated with a layer of an alloy comprising copper, tin and zinc. The alloy layer reduces hydrogen gassing within the cell and is particularly useful in cells with no added mercury. Embodiments of the invention include cells with prismatic, cylindrical and button shaped containers and cells with positive electrode active materials including manganese dioxide, silver oxide and oxygen.
Taking the above approaches into consideration, there is still a need for electrochemical cells having constructions which provide leakage resistance and include a negative electrode or counter electrode casing that serves as a current collector, and further resist discoloration, thereby contributing to a desirable aesthetic appearance of the electrochemical cells. It is also desirable for the cells to have excellent shelf life, electrical characteristics and discharge capacity.
In view of the above, an object of the present invention is to provide electrochemical cells that contain no added mercury and are less susceptible to hydrogen gassing during storage and periods of non-use than cells according to the prior art.
Another object of the invention is to provide an electrochemical cell with an electrode containing a metal such as zinc as an active material, and an aqueous alkaline electrolyte that contains no added mercury and produces little hydrogen gas during storage and periods of non-use.
Yet another object of the invention is to provide a method of making an electrochemical cell with an electrode containing a metal such as zinc as an active material, and an aqueous alkaline electrolyte that contains no added mercury and produces little hydrogen gas during storage and periods of non-use.
A further object of the invention is to provide an electrochemical cell with no added mercury and a method of making the cell, the cell having an aqueous alkaline electrolyte and a housing with a negative casing in contact with a negative electrode in which the cell is economical to produce, exhibits a low level of hydrogen gassing during storage and use, and has an attractive external appearance.
Still another object of the invention is to provide a casing for an electrochemical cell, particularly a counter electrode or negative electrode casing, having a coating including an external layer comprising copper, tin and zinc at least on the outer surface of the casing that is exposed to the ambient environment that provides the casing with resistance to discoloration.
Another object of the present invention is to provide an electrochemical cell with a negative electrode or counter electrode casing having a coating of two or more layers on a substrate including a copper inner layer, with the coating comprising a first layer having a greater weight percentage of copper than a second layer and the second layer having a greater weight percentage of tin than the first layer, wherein the cell has no added mercury, with the exposed external surface of the coating casing having a bright finish resistant to discoloration.
Still another object of the present invention is to provide a metal casing which serves as the current collector for a negative electrode or counter electrode that is post-plated with at least a first layer and a second layer on at least an exposed portion of an exterior surface of the casing wherein the first layer has a higher copper content than the second layer, and the second layer has a higher tin content than the first layer.
Yet another object of the present invention is to provide a method of making an electrochemical cell with a negative electrode or counter electrode casing comprising a plated coating having variable tin content and copper content in different layers or depths of the coating that are derived from utilizing a variable current density during plating.
A further object of the present invention is to provide a method for forming a coated casing for an electrochemical cell including utilizing multiple current densities to provide various coated layers on the casing.